This invention relates to a spectacle lens and, more particularly, to an improvement in a progressive power omnifocal lens having a zone along which the refracting power progressively changes vertically in the vicinity of the horizontal center of the lens.
A progressive power omnifocal lens is used for presbyopia, having a refracting power that gradually changes along a line on the lens (main fixation line or a principal sight line) along which the sight line passes when it shifts from far vision to near vision. The main fixation line on a progressive power omnifocal lens should be located at a position where the sight line most frequently passes. Preferably, there is no astigmatism along the main fixation line.
A point on the surface of the lens where the astigmatism is zero can be regarded as an infinitesimal spherical surface and is generally called an "umbilical point". A row of successive "umbilical points" is called an "umbilical meridian", and this umbilical meridian is usually employed as the main fixation line. The degree of astigmatism is zero on the umbilical meridian of a progressive power omnifocal lens, but it increases at an ascending rate as it deviates from this line. There are therefore extremely inferior optical conditions.
In other words, in the region of a progressive power omnifocal lens where the refracting power gradually changes, only that region which is in the vicinity of the umbilical meridian, has good optical performance (region of distinct vision). The other regions are usually treated as regions of non-distinct vision due to the existence of astigmatism.
Consequently, the disposition of the umbilical meridian (main fixation line) on a spectacle lens is very important. This line should be located at a position where the sight line most frequently passes on the basis of elaborate studies as to how the sight line of the user wearing the spectacles moves.
Examples of conventional techniques relating to the disposition of the main fixation line are disclosed in Japanese Patent Examined Publication No. 47-9626, and Japanese Patent Unexamined Publication Nos. 57-10113, 57-210320, and 59-88718. Japanese Patent Examined Publication No. 47-9626 and Japanese Patent Unexamined Publication Nos. No. 57-10113, and 57-210320 disclose techniques relating to asymmetry of the refracting surface of the lens due to the disposition of the main fixation line based on convergence of the eyes (inward movement of the sight lines of the left and right eyes when shifting from far vision to near vision). In particular, Japanese Patent Unexamined Publication No. 59-88718 discloses a technique relating to the relationship between the inward displacement of the main fixation line in response to convergence of the eyes and the changes that take place in the refracting power along the main fixation line.
These techniques, however, are not concerned with determination of the disposition of the main fixation line with respect to the addition of the lens. A person suited to wearing a progressive power omnifocal lenses of comparatively low addition is an incipient presbyopia person who is unskillful to focus the eye and whose experience in wearing far vision spectacles is inadequate. Generally, such a person is unskilled in the basic manner of using progressive power omnifocal lenses by, for near vision eye work, projecting the chin slightly forward and directing the sight lines downward so as to see through the lower portions of the spectacle lenses (for near vision eye work). He tends to look a close objects through slightly above the near vision portions of the lenses (these portions designed for intermediate-distance eye work). Complaints that "it is difficult to see a close objects" or "near-vision is restricted" are often made as a result of this tendency.
This is because looking through the portions above the proper portions set in the spectacle lenses for near vision focusing means looking with the sight lines from the left and right eyes passing through portions that inwardly deviate from the main fixation lines. As described above, this is near vision focusing with a high degree of astigmatism. Furthermore, the directions of astigmatism with respect to the left and right eyes (the directions in which the image is warped) differ from each other to such a degree that they meet almost at right angles, thereby making image fusion highly difficult.
In a possible design method aimed at avoiding this defect, the portion for near vision focusing is shifted higher in the case of a low addition lens. However, this method is the same as a method aiming at reducing the length of the zone of progressive change, resulting in another problem of producing abrupt increase in the degree of astigmatism in the lateral area.
In view of these problems, it is an object of the present invention to provide an easy-to-use progressive power omnifocal lens in which the main fixation line is disposed in accordance with the characteristic tendency with which a particular person wearing the spectacles shifts the sight line from each of his or her eyes.
To this end, the present invention provides a progressive power ominfocal lens having an area in which the refracting power progressively changes (successively increases) from upper to lower positions in the vicinity of horizontal center of the lens; and a main fixation line generally called an "umbilical meridian" formed in this progressive-change area, in which the degree of astigmatism along the main fixation line is substantially zero, and the main fixation line is gradually displaced toward the nose in accordance with the convergence of the eyes as it extends from an upper position to a lower position, the displacement of the main fixation line toward the nose being varied depending upon the addition of the lens.
The present invention also provides a progressive power omnifocal lens in which the position from which the displacement of the main fixation line toward the nose is started is shifted upward if the addition of the lens is reduced.
The addition of the progressive power omnifocal lens corresponds to the degree to which the person who will wear the progressive power omnifocal lens lacks the ability to focus the eyes.
Therefore, a progressive power omnifocal lens of comparatively low addition is used for a presbyopia person at a comparatively early stage of experiencing long-sightedness while a progressive power omnifocal lens of comparatively high strength is used for a presbyopia person at a comparatively late stage of experiencing long-sightedness.
There is a great difference between presbyopia persons at the early and late stages in terms of the manner in which the sight lines shift from far vision to near vision.
Specially, a presbyopia person at an early stage of development of this condition tends to move (downward) the sight lines to a smaller extent during the course of changing from far vision to near vision compared with a person who is experiencing a late state of long-sightedness.
In consequence, the main fixation line (indicated at 4 in FIG. 1) on a progressive power omnifocal lens of comparatively low addition (lens 1 in FIG. 1) is formed in such a manner that it extends downward from a position higher than that of the main fixation line (indicated at 6 in FIG. 2) on a progressive power omnifocal lens of a comparatively high addition (lens 5 in FIG. 2) while being displaced inward (toward the nose). The main fixation line is thus made to coincide with the movement of the sight line of a each individual presbyopia person.